Apparatus for making perforations in sheet material by electric discharge

ABSTRACT

Apparatus for making perforations in sheet material by generating electric discharge through the sheet while passing the sheet between a group of discharge electrodes and a common grounded electrode and producing a gaseous stream in any desired amounts between and around the discharge electrodes and the surface of the sheet, thereby obtaining a perforated sheet having uniform perforation spacing and density as well as descreased scorching or blackening created around the perforations in the perforated sheet.

United States. Patent 1 1 Machida et al.

[ 1 Jan. 21, 1975 1 APPARATUS FOR MAKING PERFORATIONS IN SHEET MATERIAL BY ELECTRIC DISCHARGE [75] Inventors: Chuichi Machida, Hasuda; Kozo Tsuchida, Urawa, both of Japan [731 Assignees: Sanyo-Kokusaku Pulp Co., Ltd.,

Tokyo; Mishima Paper Manufacturing Co., Ltd., Chiyoda-ku, both of, Japan 22 Filed: Feb.4,1974

21 App1.No.:439,424

Related US. Application Data [63] Continuation-in-part of Ser. No. 334,082, Feb. 20,

1973, abandoned.

[30] Foreign Application Priority Data July 17, 1972 Japan 47-70719 [52] US. Cl. 219/384, 346/74 S [51] Int. Cl. H0511 7/18 [58] Field of Search 219/69, 121 EB, 383, 384; 204/312; 346/74 R, 74 S [56] References Cited UNITED STATES PATENTS 7/1956 Dresser 219/384 2,763,759 9/1956 Gorske et a1. 219/384 3,098,143 7/1963 Warmt 219/384 3,185,896 5/1965 Gorslce eta1.... 219/383 X 3,308,050 3/1967 Denis 204/312 3,351,740 11/1967 Hewes 219/384 3,385,951 5/1968 Bancroft et al.. 1. 219/384 3,483,374 12/1969 Erben 219/383 X 4/1971 Marr, Jr

Primary Examiner-Vo1odymyr Y. Mayewsky Attorney, Agent, or Firm-Frank J. Jordan 57 ABSTRACT Apparatus for making perforations in sheet material by generating electric discharge through the sheet while passing the sheet between a group of discharge electrodes and a common grounded electrode and producing a gaseous stream in any desired amounts between and aroundthe discharge electrodes and the surface of the sheet, thereby obtaining a perforated sheet having uniform perforation spacing and density as well as descreased scorching or blackening created around the perforations'in the perforated sheet.

14 Claims, 6 Drawing Figures PRESSURE lotm Patehtedv Jan. 21, 1975 r 5 Sheets-Sheet 1 F/ GQ/A v m k w k w u A v FIGJB mm Dwmmma lot Patentd Jan. 21, 1975 5 Sheets-Sheet 5 HIGH FREQUENCY PULSE SOURCE @ONVERTER i TRANSISTOR RECTIFIER 1 AND I PULSE" DRIVER .wwwiwiiiliL APPARATUS FOR MAKING PERFORATIONS IN SHEET MATERIAL BY ELECTRIC DISCHARGE This is a Continuation-in-Part application of Ser. No.

334,082 filed Feb. 20, 1973, now abandoned.

BACKGROUND OF THE INVENTION thereto while passing the sheet through between the.

discharge electrodes and the common grounded electrode and ejecting ordrawing any desired amounts of an air-based compressed gas from or into nozzles provided around or near thedischarge electrodes to or from the discharge or thesheet. In one embodiment of this invention the discharge electrodes are each con nected to an independent or separate, high frequency pulse, high voltage power source. In another embodiment, the sheet to be perforated is passed through between a group of the discharge electrodes and the common grounded electrode without contacting the discharge electrodes.

Synthetic leather, rubber sheets, rice paper for tobacco, kraft paper for cement, polyethylene-laminated paper, plastic fiber-mixed paper, laminates of paper and synthetic resin sheets, and the like sometimes need to be provided with gas-permeability and, to this end, they have heretofore been perforated by effecting electric discharge therethrough. 'Such conventional perforating process comprises effecting electric discharge through a sheet to be perforated while passing the sheet on a metallic roll or plate and keeping it in contact with a plurality of discharge electrodes connected to a single power source which applies a relatively high frequency pulse voltage to said electrodes. However, the conventional process has disadvantages as follows.

I. Perforations made in a sheet greatly vary in diameter since the sheet is not necessarily uniform in thickness and density at all parts thereof, that is, discharge resistances at all the parts are different from each other. Further, discharge current tends to concentrate locally, for instance, at the peripheral portions of the perforations thereby scorching and damaging said portions.

2. The sheet is liable to be mechanically damaged because of the contact of the sheet with the electrodes. Such damage will have an increasingly adverse effect on the quality of the perforated sheet as the sheet used is smaller in thickness.

3. It requires a large numer of personnel and is ex-..

"and, in some cases, will betorn or cut off due to the crease. This tendency has been especially remarkable with the recent realization of the speed-up of the treatment of the sheet.

5. The space between the neighboring perforations (the space being hereinafter called perforation spacing) and the diameter of the perforations are undesirably large, and the perforation density (number of perforations per unit area) is unsuitably low.

Accordingly, an object of the present invention is to overcome the disadvantages of known prior art arrangements and to provide perforating apparatus which makes it possible to inhibit a temperature raise in the discharge portions of discharge electrodes, decrease the wear (or consumption) of the material at the tip of said discharge portions, and control perforation conditions.

Other features which are considered characteristic of the invention are set forth in the appended claims.

relationship to specific embodiments, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without department from the spirit of the invention and within th scope and range of equivalents of the claims.

SUMMARY OF THE INVENTION Apparatus for making perforations in a sheet includes a plurality of discharge electrodes and a common grounded electrode opposite thereto. The discharge electrodes and the common grounded electrode are disposed to allow the sheet to pass therebetween and to effect discharge therebetween through the sheet to produce perforations in the sheet. Nozzle means are arranged around the discharge electrodes such that the axes of the nozzles are directed perpendicularly to the sheet surface, whereby the nozzle means establish a gas stream in a direction perpendicular to the sheet surface, such gas stream passing between and around the discharge electrodes and the sheet to control perforation conditions.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A is a partial diagrammatic plan view showing the construction of discharge electrodes of an electrical perforating apparatus according to one embodiment of the invention, showing the disposition of a sheet being perforated with respect to the discharge electrodes.

FIG. 1B is a cross-sectional view taken along the lines A A of FIG. 1A.

FIG. 1C shows the variation in pressure in the transverse direction of a sheet being perforated utilizing an the gas stream drawn in the direction from the surface of the sheet-being perforated to around the discharge electrode and the variation in pressure exerted thereon by the drawn gas stream.

FIG. 4 is a schematic view of the electric circuits through which suitable electric power is supplied to the discharge electrodes according to the invention.

Although the invention is illustrated and described in DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the present invention, a natural or synthetic substance in the sheet form such as a synthetic resin sheet, paper, or a paper-synthetic resin sheet laminate, is passed on a common grounded metallic roll or plate through between a plurality of discharge electrodes usually in the needle form and the grounded metallic roll or plate positioned opposite thereto to effect electric discharge through the passing or travelling each connected to ahigh frequency pulse source via a step-up transformer to form an independent discharge circuit, and a high frequency pulse of a high voltage and small current is applied to each of the electrodes. Thus, a sharp and intense discharge is effected between the discharge electrodes and the grounded metallic roll or plate to thereby make fixedly spaced perforations over the whole width of the sheet almost at a time. Such discharge is repeated at a fixed interval of time while the sheet is travelling, thereby making perforations at a fixed space also in the travelling direction. In this process each of the electrodes is in its independent circuit and power from a power source is uniformly distributed to each of the electrodes, and electric discharge will therefore be effected'almost smoothly even if a sheet to be perforated is not uniform in density and thickness. Further, the discharge from the individual electrodes will not greatly be affected by the fact that the gap between each electrode and the sheet is not adjusted to be strictly uniform.

Adjustment or control of the flow of air or other gases ejected from the nozzles under the specific discharge circuit conditions in accordance with this invention will make it possible to conspicuously increase perforation density (number of perforations per unit area), control the frequency of perforation and the diameter of perforations to be obtained, inhibit scorching and blackening possibly caused in the peripheral portions of the perforations, inhibit a temperature raise in the discharging portions of discharge electrodes and decrease the consumption of the material of the tip of the discharge electrodes. Such adjustment, in combination with the use of an inert gas such as CO N or steam frequency, while in the process according to the present invention, electric discharge in a dynamic gas such as a forced gaseous stream composed mainly of compressed air is utilized and perforation adjustment is made by adjusting the amount of the gaseous stream ejected or drawn.

Apparatus for carrying out the present invention comprises (A) a plurality of discharge electrodes and (B) a common grounded electrode positioned opposite thereto to allow a sheet beingperforated to pass therebetween without contacting with the discharge electrodes and simultaneously generate electric discharge through the sheet for perforation, said discharge electrodes (A) being each connected via a step-up transformer to a high frequency pulse source thereby forming a separate discharge circuit to apply a high frequency pulse of high voltage and small current to each of the discharge electrodes, and (C) means for passing a gaseous stream between around each discharge electrode and the surface of a sheet to be perforated, said means (C) being composed essentially of nozzles for ejecting the gas, valves for adjusting the flow of the gas, piping for feeding the gas, a compressor or vacuum pump and Bombe (or pressure container) for gas.

Turning now to the illustrated embodiments in the drawings, FIG. 1A shows a sheet 1 which is moved upwardly with respect of the drawing as indicated by the arrow symbol. A plurality of discharge electrodes 2 are disposed above the sheet 1, in rows as indicated by the lines I-I and II-II and at a predetermined space from each other, and they are directed perpendicularly to the surface of the sheet 1. Such disposition of the discharge electrodes 2 will permit perforations 8 to be obtained at a uniform spacing from each other across the width of the sheet 1.

The reason why the discharge electrodes 2 are divisionally disposed in rows as indicated by the lines II and IIII, is as follows.

In the electrical perforation of a sheet having a fixed width, an increase in the number of rows in which discharge electrodes are disposed will permit the use of a correspondingly increased number of such discharge electrodes without unduly lessening'the space therebetween, thereby obtaining satisfactory perforations at'a smaller space from each other. If a fixed number of discharge electrodes are used, such increase in the the number of rows will permit the discharge electrodes to be disposed at a greater spacing from each other thereby increasing the insulation resistance of the sheet being perforated. Moreover, the discharge electrodes (disposed in rows 1-1 and II-II) can be supplied with voltages of opposite polarity as explained hereinafter,

- to thereby decrease the electrostatic capacity of the as the ejected or drawn gas, will further be effective in sheet between the rows I-I and II--II, and thus enabling a steep discharging pulse to be maintained.

Gas-ejecting nozzles 5 having an annular opening, are each disposed around the discharge electrodes 2 and may be provided by machining an electrode support of electrical insulating material 4 at the portion thereof opposite the sheet to form the annular opening. In FIG. 1B, the discharge electrodes 2 are positioned out to contact with the sheet 1' and have their axes directed-perpendicularly to the surface of the sheet 1. Each of the discharge electrodes 2 is connected to a different electric supplying wire 7. Between these discharge electrodes and a grounded common electrode 3 which is positioned beneaththe sheet '1, sharp and one end of the support, thereby ejecting a gas stream through the opening of a gas-ejecting nozzle and perpendicularly to the sheet surface.

FIG. 1C shows the pressure variation on the sheet surface along the width thereof which is caused by the ejection of the gas stream as shown in FIG. 13. Such mode of generating a pressure variation is explained in more detail with reference. to FIG. 2.

In FIG. 2, a pressurized gas is ejected from a pressure compartment 9 through'the nozzle 5 having an annular opening provided around the discharge electrode 2 towards the sheet surface, and the flow directions of the gas stream are-indicated by the arrows. By such ejection of the gas, there is'a variation in pressure exerted on the sheet surface as shown in FIG. 2, and thus the discharging arclO is enveloped with the gas stream. The gas stream may partially flow into the discharging arc, but in this case, the amount of gas supplied should be adjusted so that the aforementioned partial flow does not extinguish the arc beneath the discharge electrode. 7

By providing the gas ejection in such a manner as above, the path for the discharging arc is defined or confined to the spot of the sheet surface which is positioned just beneath the discharging electrode, and thus the gas stream prevents the discharging are from expanding in all directions in the form of creeping discharge and thereby producing perforations only at said spot.

The necessity for preventing the discharging are from expanding in all directions will be concretely explained with reference'to FIG; 1A.

The points of the sheet 1 which are directly beneath the mid points between the neighboring ones of the dis charge electrodes in the row II have already been perforatedby the discharge electrodes in the row IIII. Moreover, at the points of the sheet 1 which are downstream of the points just beneath the discharge electrodes in the row II, there have already been perforations made by the discharge action of the discharge electrodes in the row II.

Thus, if the gas stream is not produced, the discharging arcs from the discharge electrodes in the row II will pass through these points that have been already perforated and thus have a low insulating resistance. Accordingly, there may hardly be produced new perforations at the points that are positioned directly beneath the discharge electrodes when discharged. Such inclination or tendency becomes'more conspicuous or pronounced when the sheet is of the type having more insulating resistance or when greater perforation density is desired by making narrower the spacing between the perforations to be produced.

As an alternate arrangement the discharging arc can be controlled by drawing a gas stream instead of by ejecting the gas stream to thereby conduct an efficient perforation.

FIG. 3 shows a gas stream generated by the suction of gas and also illustrating the variation in pressure exerted on the sheet surface by the stream. In this case, unlike the case of FIG. 2, the annular opening is defined by the discharge electrode and an annular projection 11 is so constructed that the gas passes through the narrow gap between the projection 11 and the sheet surface to thereby generate a gradient of pressure exerted on the sheet surface as shown in FIG. 3. Since the pressure compartments 9 are maintained below atmospheric pressure, the pressure variation or gradient in this case is more remarkable than that in FIG. 2, and the gas stream flows in the direction opposite to the path for creeping discharge which might otherwise be generated along the sheet surface. Thus, the gas stream generated by suction has a more positive inhibiting effect on the formation of paths for creeping discharge and obtains amore efficient perforation than the gas stream generated by ejection.

If desired, the gap between the annular projection 11 and the sheet surface may be made smaller so that the sheet travels in sliding contact therewith, thereby further enhancing the inhibiting effect.

The electric circuit shown in FIG. 4 and through which electric power is supplied to the discharge electrodes is constructed in view of the fact that a synthetic resin sheet being perforated has a particularly high insulating resistance anad is thus difficult to perforate. The circuit comprises a high frequency pulse source and a plurality of step-up transformers, and has characteristics as follows:

l. The step-up transformer is of a high inductance type which is effective in the rapid control of electric current and in which is composed of a secondary winding which supplies a high voltage of a steep wave form when a pulse voltage is applied to a primary winding. In actual practice, an ignition coil usually for use with automobiles may be used as such. Such ignition coil is of a small size, inexpensive, of a high clurability and easily available in quantities.

2. The secondary winding of .each step-up transformer may be connected to a single discharge electrode. Thus, there are necessary step-up transformers of the same number as the discharge electrodes, so that respective discharging circuits are formed independently to enable a plurality of discharge electrodes to discharge simultaneously. and securely.

3. It is possible to reduce both the electrostatic capacity between lines I--I and Il-Il and the earth capacity. As shown in FIG. 4, the lines l-I and IIII from the secondary winding of the step-up transformer are connected to the discharge electrodes in the rows 1-] and IIII, respectively, as shown in FIG. 1A, that is, the step-up transformer consists of a pair of ignition coils and the secondary winding is center-tapped and grounded and has the opposite ends connected -to the discharge electrodes, thus applying to the discharge electrodes voltages of opposite polarity. These lines I-I and .II-II comprise feeders having equipotentials, respectively and they effect a power supply of opposite polarity. Thus, enough spacing of the lines I-I and IIII from each other, and enough spacing of these lines and the earth from each other, will greatly reduce the electrostatic capacity between the lines and between the same and the earth,

thereby allowing a voltage of steep wave form gen-' erated in the secondary winding to be applied to each of the discharge electrodes.

lowing examples.

4. The high frequency pulse source'is controlled by means of transisters to supply a predetermined pulse to the-primary winding of the step-up transformer. This pulse source does not include any mechanically driven parts, and it can widely vary the I pulse voltage and frequency.

As previously mentioned, the conventional processes give perforated sheets having a large perforation space, asmall perforation density, perforations of large diameter. and remarkable scorching and blackening produced in the peripheral portion of the perforations, and this tendency is greater as sheets used have higher di electric strength. More particularly, once the travelling sheet is locally destructively perforated by effecting discharge therethrough, the discharge concentrates in said destructively perforated portions due to their rapid decrease in electric resistance and further this concentration continues for such a time that the sheet takes to travel as far as the creeping discharge is interrupted, whereby the current concentrates in said destructively perforated portions and consequentially a severer scorching or blackening is produced; while perforation pauses at other portions than the destructively perforated ones in the sheet thereby obtaining an undesirably large perforation spacing.

As is seen from the foregoing, the process of this invention which eliminates the above-mentioned drawbacks, is very effective in obtaining a reduced perforation spacing, small diameter of perforations and minimized scorching'in the peripheral portions of the perforations by passing a forced gaseous stream through the I arc to interruptthe creeping discharge on the surfaces of the'sheet and properly control the thermal energy in the discharge. This process also makes it possible to controllably vary the perforating conditions by adjusting the flow rate of the gaseous stream.

The following Table 1 shows the effects or advantages obtained by the practice of this invention in comparison with those obtained by the conventional pro- 40 Table l Electric Electric Type of discharge discharge in discharge in forced gaseous stationary air stream Main method for Amount of air Applied voltage, adjusting ejected current and perforation frequency Perforation spacing Small Large Diameter of perforations Small Large Scorching or blackening Small Large of peripheral portions 1 of perforations Wear of the tip of Small Large discharge electrodes Temperature raise in the Small Large discharge portionof electrodes Range of variation of Small Large permeability (or porosity) of perforated article This invention will be better understood by the fol- EXAMPLE As discharge electrodes there were used a plurality of tungsten wires arranged at an approximately 3-mm space. The discharge electrodes were disposed opposite to a common grounded metallic roll so that the former were several millimeters spaced from the latter, and each of the discharge electrodes was connected to a high-voltage power source capable of generating high frequency pulses. Nozzles for ejecting or drawing a gas were provided around each tungsten wire so that a stream of the gas surrounded arcs generated between the discharge electrodes and metallic roll and the flow rate of the gas was made uniform by the use of a flowadjusting valve, thereby allowing the arc discharge to continue at every part of a sheet to be perforated. The sheet was travelled at a velocity of m/min and a discharge frequency of 3 KHZ while it is in contact with the grounded metallic roll and out of contact with the tungsten wires, that is, needle-like discharge electrodes. The gas used was a pressurized air. Table 2 shows a comparison between the results obtained from the test using such gaseous stream and those obtained without the use of a gaseous stream as mentioned above.

Table 2 New process Conof this ventional invention process Sheet to be Perforating condi- Electric Electric perforated tions and properties discharge discharge of product in forced in gaseous stationary stream air Discharge voltage 7 7 (KV) Ejecting pressure at nozzle (Kg/cm 1.5 0 Wrapping paper A Perforation density (No, of perforations v (Weight per unit area) 50-60 10 -20 37g/m Diameter of perforationS (it) 30-40 so-roo Blackening at the peripheral portion Slightly Remarkably of perforations black black Discharge voltage l5 l5 Ejecting pressure at nozzle (Kg/cm) 2.0 0 Wrapping paper Perforation density (No. of perforations (Weight per unitarea) 20-30 S-7 l0Og/m 1 Diameter of perforations (u) 50-80 l00300 Blackening at the peripheral portion Slightly Remarkably of perforations brown brown Discharge voltage 15 15 Soft Ejecting pressure polyvinyl at nozzle (Kg/cm 2.0 0 chloride Perforation density 15-20 5-7 (Thickness 300p.) Diameter of perforations (11-) 50-l00 300-500 Blackening at the peripheral portion Slightly Remarkably of perforations dark brown dark brown What is claimed is: I 1. An apparatus for the perforation of a sheet com prising a plurality of discharge electrodes electrically insulated from each other, a common grounded elections in the sheet, annular nozzle means disposed around and surrounding eachof said discharge electrodes, each of said annular nozzle means having their axes perpendicularly tothe sheet surface, means for es-- I tablishing a gas stream, said nozzle means forming said gas stream in a direction perpendicular to the sheet surface, said gas stream passing between and around each of said discharge electrodes and the sheet to control perforation conditions.

2. An apparatus as claimed in claim 1, wherein said means for establishing a gas stream includes a compressed gas supply, said nozzle means being connected to said compressed gas supply, therebyejecting a compressed gas stream in a direction from the nozzles perpendicularly to the sheetsurface.

3. An apparatus as claimed in claim- 1, wherein each of said nozzle means includes a nozzle having an annular projection at the peripheral portion thereof opposup transformer to said high. frequency pulse source to form an independent discharge circuit thereby applying a high frequency pulse of high voltage and small current to said discharge electrode.

5. An apparatus as claimed in claim 4, wherein said step-up transformer comprises a pair of ignition coils, a primary winding connected to the high frequency pulse source, and a center-tapped secondary winding, the opposite endsof said secondary winding being connected to a pair of said discharge electrodes whereas said cente r-tapped secondary winding is grounded, thus applying to said pair of discharge electrodes voltages of opposite polarity.

6. An apparatus as claimed in claim 3, including a step-up transformer and a high frequency pulse source, said discharge electrodes being connected via said stepup transformer to said high frequency pulse source to form an independent discharge circuit thereby applying a high frequency pulse or high voltage and small current to said discharge electrode 7. An apparatus as claimed in claim 6, wherein said step-up transformer comprises a pair of ignition coils, a primary winding connected to the high frequency pulse source, and a center-tapped secondary winding, the opposite ends of said secondary winding being connected to a pair of said discharge electrodes whereas said center-tapped secondary winding is grounded, thus applying to said pair ofdischarge electrodes voltages of opposite polarity.

8. An apparatus as claimed in claim 1 wherein each of said nozzle means is constructed and arranged to direct said gas stream in a manner to control the perforation spacing, perforation density, the diameter of the perforations formed, and any blackening and scorching which may be produced at the peripheral edge portions of the perforations.

9. An apparatus as claimed in claim 1 wherein said gas stream includes an inert gas to prevent scorching and blackening.

10. An apparatus as claimed in claim 1 wherein said cumscribing relationship about individual discharge electrodes.

13. -An apparatus as claimed in claim 1 wherein each of said nozzle means includes a nozzle comprising a gas discharge passage having an axis perpendicular to said sheet, each of said discharge nozzles being centrally disposed within one of said discharge passages. 14. An apparatus as claimed in claim 1 wherein each of said nozzle means includes a nozzle having a toroidal opening circumscribing said discharge electrode. 

1. An apparatus for the perforation of a sheet comprising a plurality of discharge electrodes electrically insulated from each other, a common grounded electrode opposite thereto, said discharge electrodes and common grounded electrode being arranged to allow the sheet to pass therebetween and effect discharge therebetween through the sheet to produce perforations in the sheet, annular nozzle means disposed around and surrounding each of said discharge electrodes, each of said annular nozzle means having their axes perpendicularly to the sheet surface, means for establishing a gas stream, said nozzle means forming said gas stream in a direction perpendicular to the sheet surface, said gas stream passing between and Around each of said discharge electrodes and the sheet to control perforation conditions.
 2. An apparatus as claimed in claim 1, wherein said means for establishing a gas stream includes a compressed gas supply, said nozzle means being connected to said compressed gas supply, thereby ejecting a compressed gas stream in a direction from the nozzles perpendicularly to the sheet surface.
 3. An apparatus as claimed in claim 1, wherein each of said nozzle means includes a nozzle having an annular projection at the peripheral portion thereof opposing the sheet surface, a suction means, said annular projection being connected to said suction means, thereby generating a drawn gas stream between the sheet surface and the discharge electrodes.
 4. An apparatus as claimed in claim 2, including a stepup transformer and a high frequency pulse source, said discharge electrodes being connected via said step-up transformer to said high frequency pulse source to form an independent discharge circuit thereby applying a high frequency pulse of high voltage and small current to said discharge electrode.
 5. An apparatus as claimed in claim 4, wherein said step-up transformer comprises a pair of ignition coils, a primary winding connected to the high frequency pulse source, and a center-tapped secondary winding, the opposite ends of said secondary winding being connected to a pair of said discharge electrodes whereas said center-tapped secondary winding is grounded, thus applying to said pair of discharge electrodes voltages of opposite polarity.
 6. An apparatus as claimed in claim 3, including a step-up transformer and a high frequency pulse source, said discharge electrodes being connected via said step-up transformer to said high frequency pulse source to form an independent discharge circuit thereby applying a high frequency pulse or high voltage and small current to said discharge electrode.
 7. An apparatus as claimed in claim 6, wherein said step-up transformer comprises a pair of ignition coils, a primary winding connected to the high frequency pulse source, and a center-tapped secondary winding, the opposite ends of said secondary winding being connected to a pair of said discharge electrodes whereas said center-tapped secondary winding is grounded, thus applying to said pair of discharge electrodes voltages of opposite polarity.
 8. An apparatus as claimed in claim 1 wherein each of said nozzle means is constructed and arranged to direct said gas stream in a manner to control the perforation spacing, perforation density, the diameter of the perforations formed, and any blackening and scorching which may be produced at the peripheral edge portions of the perforations.
 9. An apparatus as claimed in claim 1 wherein said gas stream includes an inert gas to prevent scorching and blackening.
 10. An apparatus as claimed in claim 1 wherein said discharge electrodes have discharge ends spaced from said sheet.
 11. An apparatus as claimed in claim 3 wherein said annular projections on said nozzles slidably contact said sheet.
 12. An apparatus as claimed in claim 1 wherein each of said nozzle means includes a nozzle arranged in circumscribing relationship about individual discharge electrodes.
 13. An apparatus as claimed in claim 1 wherein each of said nozzle means includes a nozzle comprising a gas discharge passage having an axis perpendicular to said sheet, each of said discharge nozzles being centrally disposed within one of said discharge passages.
 14. An apparatus as claimed in claim 1 wherein each of said nozzle means includes a nozzle having a toroidal opening circumscribing said discharge electrode. 